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15° W when the bow is turned to west as shown in Fig. 9, or, the deviation may be several degrees westerly on one heading and easterly on a different heading. This is due entirely to the position of the compass in relation to the iron and steel of the ship. Thus, in Fig. 9, suppose that a represents an engine, or

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other large mass of iron, located in the forward part of the vessel. This mass of iron will attract the compass needle, but its effect will be different for different headings. When the ship is heading magnetic north, for example, the disturbing force a will act in the same direction as the directive force of the

magnetic needle and the deflection on that heading will be a minimum. When the ship is heading northwest, as shown in the center diagram, the disturbing force a will act at an angle and cause a deflection. When the ship is heading west, as in the left-hand diagram, the deflection, or deviation, of the compass will be at its maximum, because in this position the

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mass is at right angles to the magnetic meridian or the normal direction of the needle. It is evident, therefore, when a compass is placed on board of a vessel and surrounded by masses of iron and steel, it becomes unreliable because of the influence of this metal on the magnetic needle.

16. To guard against the effect of deviation, certain manipulations are employed to reduce mechanically the error to a minimum value and restore, as far as can be done, the compass to its original function. This process is known technically as compensating the compass, and consists chiefly in placing suitable magnets in such position about the compass as to counteract the magnetism of the surrounding iron. Thus, if the vessel is heading west, as in the first diagram of Fig. 9, and the compass has a westerly deviation, a magnet b may be placed so that it will counteract the effect of the mass a and bring the needle back to its normal position. One, two, or more magnets may have to be used for this purpose and the vessel headed in different directions before the deviation is reduced to its least value. But even after being carefully compensated, errors may remain that may impair the usefulness of the compass.

17. Swinging For Deviation.-To find and tabulate the errors that remain after the compass has been mechanically adjusted with counteracting magnets a process of swinging the vessel is resorted to. The vessel is first laid in a north-and

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DEVIATION CARD

Bridge

Compass of

Steam Gacht "Peregrine Steel Vessel

Adjusted at Boston, Mass.

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south direction, according to permanent landmarks, and then swung successively with her bow on each of the thirty-two points of the compass; then, as the vessel is steadied on each point, the deviation is carefully noted and tabulated on a card

similar to the one shown in Fig. 10. This process of compensating the compass and tabulating the remaining deviation should always be made by a professional compass adjuster. The necessity of having the compass carefully adjusted is too great to leave the adjustment in the hands of incompetent persons. Deviation, like variation, is named east or west, according as the compass north is deflected east or west of the magnetic north.

18. Magnetic Property in Iron and Steel Vessels.-In the foregoing, to show why the deviation is different on different headings, an engine situated in the forward part of the vessel was used. The steel or iron of which the vessel is constructed may act in an exactly similar manner. When a vessel is built of iron, she acquires magnetism partly through induction from the earth and partly through the hammering and riveting necessary when constructing the hull. In conformity with laws of magnetic phenomena, this magnetism, or polarity, is concentrated in two poles, a negative and a positive, located in different parts of the vessel and the location of these poles will depend on the direction in which the vessel is built. For the sake of convenience and clearness, positive polarity is designated as red, and negative polarity as blue.

19. For example, a yacht built of steel or iron in a northand-south direction, with her bow toward north, will have red, or positive, polarity in her forward part, and blue, or negative, polarity in her stern. The effect of this will be that the magnetism in the yacht's forward part will repel the north end of the compass needle and, similarly, the magnetism in the after part will repel the south end of the needle. If the vessel is built in a northeasterly direction, red, or positive, polarity will be found on the port bow, and blue, or negative, polarity on the starboard quarter. Again, should the vessel be built in the direction of the meridian with her bow toward the south, her stern will possess red polarity and the bow blue polarity. When built in an east-and-west direction, the side of the yacht that faces the north magnetic pole will acquire red, and the other side blue polarity. When built with her bow to south

east or southwest, the red polarity will be on the port quarter in the former case, and on the starboard quarter in the latter case.

20. Effect of Vessel's Magnetism on Compass.-The effect of this acquired magnetism on the compass is shown in Fig. 11 and explains more clearly why the deviation is not the same on all headings. Take the case of a vessel that was built with her bow toward magnetic south and as a consequence

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has blue, or negative, polarity B in the bow and red, or positive, polarity R in the stern. Then, when heading north, or in the direction of the magnetic meridian, as in (a), there will be no deflection of the compass, because both magnetic poles of the yacht are in line with the directive force controlling the needle. If the yacht is swung with her bow northwest, the north-seeking end of the needle will be attracted by the blue polarity in the

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